Known examples of x-ray marker devices include registration bodies, so-called fluoroscopic registration kits. These are fixedly connected to the image intensifier of a C-arm. Some enable a rotation relative to the image intensifier. Optical markers (navigation markers) are also attached to the registration bodies and allow the position of the registration body to be determined by means of a navigation system. The navigation systems are used for so-called image-guided navigation (image-guided surgery). The optical markers are for example detected by means of cameras which are components of the navigation system. Evaluating an x-ray image and the x-ray markers imaged in it and knowing the position of the registration body relative to the x-ray apparatus allow an instrument to be registered in a desired reference system which can be predetermined by the navigation system and allow the instrument to be displayed virtually in the x-ray image, without taking an x-ray recording.
The following documents relate to such methods or devices: U.S. 61/054,187; DE 199 17 867 A1.
If the registration body can be rotated, then there are only the options of rotating and mirroring when evaluating the x-ray image in order to achieve an assignment between the image markers and the x-ray markers. The option of mirroring is available since the x-ray image can be output in a mirrored or non-mirrored form by the x-ray apparatus. Due to this low number of possible variations (rotating and mirroring), an assignment is obvious. An assignment between the x-ray markers and the image markers can thus be performed simply and quickly by means of an algorithm.
For the assignment, information concerning the x-ray beam imaging geometry is then for example determined in accordance with the principles of the pinhole camera. This information in particular allows the position of an x-ray source to be calculated relative to the registration body.
The x-ray beam imaging geometry can for example be defined via a spatial transformation into a centre of projection (six external imaging parameters: three rotational, three translational; and four internal imaging parameters: two scaling factors which convert global coordinates [mm] into pixels of the electronic image, and the coordinates of the (computational) principal point).
The principle described above also applies in principle to the scout view (see further below). With respect to the principal point, the two coordinates (u, v) are preferably taken as the calculated centre point of the image.
The data which contains the above information concerning the x-ray beam imaging geometry is referred to here as the imaging geometry data. The imaging geometry data in particular comprises the calculated projection matrix. The imaging geometry data, and in particular the projection matrix, represent a general law of imaging for the given x-ray beam imaging geometry. They thus allow a calculation of how an arbitrary point in space will be displayed in the (undistorted) x-ray image, assuming the same x-ray beam imaging geometry as obtained when the x-ray image—which displays an image of the x-ray marker device—was produced. It thus represents a generalization of the imaging process which assumes the specific scenario of imaging the x-ray marker device onto the x-ray image with a given x-ray beam imaging geometry.
Imaging geometry data is for example calculated by means of using a camera model which reflects the actual imaging characteristics as well as possible. The usual camera model is the pinhole camera model, i.e. the imaging geometry data is for example calculated on the basis of the principles of the pinhole camera. Reference is made in this respect to the following publications, which are hereby incorporated into the disclosure by reference:
1. “An Efficient and Accurate Camera Calibration Technique for 3D Machine Vision”, Roger Y. Tsai, Proceedings of IEEE Conference on Computer Vision and Pattern Recognition. Miami Beach, Fla., 1986, pages 364-374.
2. “A Versatile Camera Calibration Technique for High-Accuracy 3D Machine Vision Metrology Using Off-the-shelf TV Cameras and Lenses”, Roger Y. Tsai, IEEE Journal of Robotics and Automation, Volume RA-3, No. 4, August 1987, pages 323-344. See also at http://www.cs.cmu.edu/˜rgw/TsaiDesc.html
3. Publication by Ziv Yaniv, “Fluoroscopic X-ray Image Processing and Registration for Computer-Aided Orthopedic Surgery”.
As may be gathered from citation No. 2 given above and the accompanying internet address, five internal imaging parameters are mentioned in the “Tsai's Camera Model” described in said document, and are referred to in said document as “internal parameters”. However, only four internal imaging parameters were mentioned above. This is due to the fact that an undistorted image is assumed in the given scenario, and the parameter “kappa1” is therefore not required. If the image were to be distorted, a rectification would preferably have to be performed beforehand (see also discussion further below). In general, the internal imaging parameters describe how the camera, i.e. the x-ray apparatus in the given scenario, forms an image, while the external parameters describe the position of the x-ray apparatus (its location and orientation) in the global coordinate system. In accordance with the invention, the Tsai Camera Model is adapted to the particular conditions of the x-ray beam imaging, as described in particular in the publication by Ziv Yaniv (see above). The two scaling factors f and/or 1/f and sx can in particular be used.
If the imaging geometry data has been determined, it is then possible to check, using a virtual x-ray image of the registration body, whether the assignment is correct. The virtual x-ray image is determined by the imaging geometry data.
If, however, the relative position between the image intensifier and the registration body is unknown, then there are a greater number of possible assignments between the x-ray markers and the image markers, which can lead to a significant computing time of a number of minutes or hours (depending on the number of markers). In daily practice, however, such a period of computation is unacceptable. Due to this long computing time, registration bodies are in practice currently mounted fixedly relative to the image intensifier.
Another example of x-ray marker devices is “x-ray calibration phantoms” (cf. DE 102 15 808) or “x-ray grids” (cf. EP 08 156 293.6 or U.S. 61/054,187) which can be used to determine calibration information for calibrating a 3D CT x-ray apparatus, in particular used to determine the position of a 3D x-ray measurement volume relative to a reference system which is predetermined by the navigation system. The x-ray marker devices, in particular the support (for example, side walls) for the x-ray markers, and the x-ray markers can be optically opaque.